Understanding Wing Technology

Discussion in 'Hydrodynamics and Aerodynamics' started by Doug Lord, Sep 18, 2010.

  1. Skyak
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    Skyak Senior Member

    That leads right up to a question I had. In the video, at around 3:20 he talks about 35 deg angle of attack from two elements, and the key being a flap on the tail of the forward element acting like a nozzle.
    -does he mean that they are bending the attached flow 35 deg with just two elements?

    I suggested there was value to doing this and was dissuaded by some very knowledgeable people.
     
  2. petereng
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    petereng Senior Member

    The flap has two purposes a) to control twist and b) to control the flow of air onto the rear element. Flaps are not nozzles. The aim is to keep the flow attached to all elements and bend the air the most. As a single foil you get about 2 max lift coefficient for the area as a good 2 elemnent wing you get a Cl=3.5.
    There are lots of videos and info on this type of thing around if you look. I'm not an aerodynamist and some of these things get complicated (I'm a mech engineer and design structures of the elements and the mechanisms that run the elements) . You can have leading edge flaps, trailing edge flaps, cascades all achieve more lift at the cost of complexity and more drag. There are a lot of things to trim on a wing and if you have extra flaps and don't trim them right you probably end up with a poorer performance. If you look at the first AC72 Artemis had a very complex wing, Oracle was in the middle and THNZ had the simplest. Cheers Peter S
     
  3. tspeer
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    tspeer Senior Member

    The flap is hinged ahead of the trailing edge of the main element & tab. The flap leading edge will move forward a little and generate some overlap when it deflects. It's possible there is some interference between the tab and flap and the flexible flap deforms and pops through on a tack, but there can't be very much interference or it would never go through.

    It's too bad the explanation he gives in the video is wrong. He compares closing the gap with the tab to a hose nozzle and says the flow through the gap accelerates when the tab closes the gap down. In fact, the opposite happens - the flow through the gap is slower with a smaller gap than it is with a wider one, at least with typical wing/flap geometries. The hose and wing gap situations are very different because the mass flow from the hose is approximately constant, while closing the gap significantly reduces the mass flow through the gap.
     
  4. Skyak
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    Skyak Senior Member

    I get the problem with using the term 'nozzle' but I still see value in trading high side pressure for lee side momentum to delay stall for max lift. I have seen it in other videos and the little AC guys have demonstrated value in it. What do we call this?

    The only caveat I see is that little AC is area constrained, so adding a third element that only increased performance down wind would be taking area from upwind plan form. This still brings me back to value in managing the slot.
     
  5. Skyak
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    Skyak Senior Member

    Peter,
    I'm an ME with limited aero too and the only part I don't grasp of your statement above is a) control twist with the trailing edge flap on the forward element. I only see that insomuch as it contributes to b) attachment. Am I missing something?

    Struggling for a better explanation (than nozzle) for what is going on I propose this:

    to increase maximum CL, a trim tab on the leading element adds asymmetry and thus camber. Due to favorable pressure created by the aft element this is more effective than increasing camber with the aft element. Since the class is area constrained, adding elements for downwind would add drag or take area from the upwind configuration.
     
  6. Doug Halsey
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    Doug Halsey Senior Member

    The slot effect & how flaps work has been discussed earlier (in this thread somewhere, I believe). AMO Smith's 1975 Wright Brothers Lecture is often cited as the definitive reference, but the paper is very long & can be difficult to follow.

    I recently came across some class notes by W. H. Mason that provide a good summary : http://www.dept.aoe.vt.edu/~mason/Mason_f/HiLiftPresPt1.pdf

    His description of Smith's paper starts on p.34, and summaries of 5 important effects of using multiple elements are on given pp.40-44.
    1. The slat effect
    2. The circulation effect
    3. The dumping effect
    4. The off-the-surface pressure recovery effect
    5. The fresh boundary layer effect

    I think the myth of the nozzle effect possibly derives from #3 &/or #5.
     
  7. Skyak
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    Skyak Senior Member

    Hi Doug, thanks for the info and stopping by. I have read and understood the explanation of 2 element slot behavior in this forum -it had 5 points too so I presume it is the same though I have not read and compared yet. It's all logical and I accept the explanation.

    Now we have this video. I can accept that the "nozzle" explanation is an unfortunate mistake, but I (we) are still left to explain or comprehend why this solution works for the little AC.

    "Struggling for a better explanation (than nozzle) for what is going on I propose this:

    to increase maximum CL, a trim tab on the leading element adds asymmetry and thus camber. Due to favorable pressure created by the aft element this is more effective than increasing camber with the aft element. Since the class is area constrained, adding elements for downwind would add drag or take area from the upwind configuration."
    09-30-2016 10:34 AM

    So what do you think of my first whack at it?

    Even if it is false, we have to admit that the concept of a nozzle would intuitively lead to this winning solution. We should be able to get the same intuition from our true understanding even if it is not as direct.
     
  8. tom28571
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    tom28571 Senior Member

    Like some others here I am out of my element in aerodynamics but completely discounting any "nozzle" effect seem premature. Would not the air speed at the slot exit depend on the pressure at the inlet? Would the pressure at the inlet depend on the foil speed and angle of incidence? I am used to considering air as non compressible at sailboat speeds but at some velocity, it is very compressible.

    Looking at a champion STOL aircraft with both leading and trailing edge flaps/slots is interesting even if there is no acceleration in the slot. Or, is there?
    https://www.youtube.com/watch?v=5htZw74UHms Home town boy making good.
     
  9. Doug Halsey
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    Doug Halsey Senior Member

    The nozzle explanation is completely inappropriate because it implies that there is a source of fluid at a higher total pressure than the oncoming flow, and that it is being accelerated by forcing it through a constriction.

    In contrast, none of the flow coming through a multielement airfoil's slot has a total pressure any higher than the original freestream flow, and some of it (coming from the boundary layer on the forward element's windward surface) is actually less. In addition, the flow is not forced to go through the slot; it has an alternate path it can take, staying to windward of the downstream elements.

    The closest thing to the nozzle effect is item #5 in the above list, the fresh boundary-layer effect, which allows the boundary layers on downstream elements to start over with the same total pressure as the original flow approaching the upstream element, rather than the dissipated value it would have if the slot were closed.

    The flow is not being forced through the slot; it can just as easily take an alternate path. Just like the flow elsewhere, the speed at every point in the slot is affected by the flow everywhere else, not just at the "inlet" & "exit".

    There is no need to consider compressibility effects at even the fastest conceivable sailboat speeds.

    The STOL aircraft makes use of a higher energy "jet" of air from the propeller, which the flaps deflect downward to augment the lift. That's a very interesting subject, but it's not relevant to sailboats.

    Coincidentally, Smith's Wright-Brothers lecture, mentioned above, discusses power-augmented lift at some length, including some of my early work on thin jet-flaps.
     
  10. Doug Halsey
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    Doug Halsey Senior Member

    I think perhaps you're trying to squeeze too many difficult or subtle effects into too short a space. The explanations given on PP. 40-44 of the link I cited in Post #291 do a good job of explaining each of the 5 slot effects, but they have already been pared down to the bare minimum & I would be reluctant to shorten them further.
     
  11. Skyak
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    Skyak Senior Member

    We won't attract anyone knowledgeable to the conversation resurrecting the "nozzle" -and don't try to bring compressible flow to sails. That dead horse was beaten to a smooth puddle of muck, then beaten some more for so long it dried into a fine powder, which was beaten vigorously until it floated away in a cloud of dust.

    That STOL competition was very impressive -down south guy comes up to the land of the bush pilot and shows them how it's done. But that configuration is consistent with the slot effect theory we have -no "nozzle".
    This little AC design -well as Ricky said to Lucy "you got some splaining to doooo".

    Maybe we should start with some simpler questions. Is it just me or is bending an air stream 35 deg with two elements that are reversible a major accomplishment?
     
  12. tom28571
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    tom28571 Senior Member

    My lack of education in aerodynamics limits detail understanding of this argument and I would like to believe that you all are completely correct but I am about to experience some of natures variation in air pressure first hand.

    The claim that the pressure at an inlet cannot exceed the free air pressure seems counter to observation. Maybe, but maybe you guys don't have theh complete story either.

    One thing I am certain of is that the law of conservation of energy applies to all these cases. When we imagine or draw a bubble around any sail system, all sails whether soft or hard, extract energy from the air flow by slowing it down and/or changing its direction.
     
  13. Doug Halsey
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    Doug Halsey Senior Member

    There are 3 different angles to consider here & it isn't always clear which one the speaker is referring to (possibly because of sloppy translations?).

    At about 02:25 in the video, he refers to the wings "latching onto streams of air at higher angle-of-attack". He then mentions a "lateen sail" reaching 20 degrees.

    Then, in discussing wings, he mentions a "wing section which can latch on to about 35 degrees apparent wind." This would make sense if he meant angle-of attack, instead of apparent wind, since the use of a flap can delay the stall to a higher angle of attack & allow higher lift, but larger apparent wind only indicates that a boat is either pointing lower &/or going slower.

    I don't think he mentions the flap angle, which I'm assuming you are referring to as the bending of the air stream.
     
  14. Doug Halsey
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    Doug Halsey Senior Member

    Once again, there is no inlet.

    And notice that I was referring to the total pressure (or equivalently, to the stagnation pressure), which is the sum of the static pressure and the dynamic pressure.
     

  15. Skyak
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    Skyak Senior Member

    Everything else above is consistent with my understanding -but there is nothing that would lead me to put a flap at the end of the first element.
     
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